The present invention generally relates to echo cancellers, and more particularly to an echo canceller which is used in a long-distance telephone system, a telephone conference system, a video telephone conference system and the like.
The echo canceller was developed for the purpose of preventing quality deterioration of communications caused by an echo which occurs in long-distance telephone lines such as satellite communication and submarine cable communication. The echo canceller technology is presently applied in a two-way relay for call diversion service, a relay for telephone conference and the like in order to improve the singing margin. In addition, the echo canceller technology is also applied in a high-efficiency encoder which has a long encoding delay so as to improve the efficiency of the transmission line.
Furthermore, in the case of the video telephone system and a loudspeaker telephone, the sound output from a speaker reflects off the walls of the room and mixes in from a microphone to cause an echo. Hence, the echo canceller is used to eliminate this echo.
A description will now be given of the echo phenomenon which occurs in the long-distance telephone line and the operating principle of the conventional echo canceller.
FIG. 1 is a diagram for explaining the echo phenomenon. Generally, in the long-distance telephone line, the subscriber lines on both ends of the line respectively employ the two-wire circuit for making transmission and reception using two lines. The long-distance telephone line which connects the two subscriber lines employs the four-wire circuit for making transmission and reception using mutually independent lines. Hybrid transformers 41 and 42 are used for the conversion from the two-wire circuit to the four-wire circuit. However, unmatched impedance at the connection point between the two-wire circuit and the four-wire circuit causes a portion of an input signal on the receiving side to leak to the transmitting side via the hybrid transformers 41 and 42. This signal portion which leaks to the transmitting side is generally referred to as an echo.
In the case of a domestic communication, the propagation time of the audio signal is short and the time difference between the transmitted signal and the echo which returns via the connection point on the receiving side is only in the order of 50 msec at the maximum. For this reason, the echo sounds as a sidetone to the speaker and no practical problems are introduced.
On the other hand, in the case of an international communication which uses the satellite line, for example, the propagation time of the going and returning signals between the two parties is approximately 300 msec or more. For this reason, the speaker must talk while listening to his own voice which is returned as echo, and this echo makes it difficult for the speaker to smoothly continue the telephone call.
The echo canceller is designed to suppress this echo, and FIG. 2 shows the operating principle of the conventional echo canceller. Echo cancellers 43 and 44 are arranged respective ends of the four-wire circuit. The echo canceller 43 includes an echo replica generator 431 for generating an echo replica signal by estimating the echo path characteristic and a subtractor 432 for subtracting the echo replica signal from the actual echo signal. Similarly, the echo canceller 44 includes an echo replica generator 441 for generating an echo replica signal by estimating the echo path characteristic and a subtractor 442 for subtracting the echo replica signal from the actual echo signal.
Generally, the echo replica generators 431 and 441 respectively estimate the impulse response of the echo paths using a transversal filter. In order to cope with the change in the impulse response, the filter coefficients of an adaptive filter are usually updated adaptively using the received signal. The adaptive filter in most cases employs an adaptive algorithm in conformance with the learning identification method because a relatively good characteristic can be obtained using a simple hardware structure. In other words, the filter coefficients of the adaptive filter are successively updated so that the square of a residual echo approaches zero.
However, when the learning identification method or a similar adaptive algorithm is used, the estimated impulse response (tap coefficients of the filter) becomes large if a narrow band signal such as a single frequency signal and a low bit rate modem signal obtained by frequency modulation is input. In this case, a divergence phenomenon occurs because a number of bits exceeds a limit determined by the hardware structure.
On the other hand, when the echo canceller is applied to the actual line, the situation is not limited to the case where the line is connected and the echo path exists, but also to a case where the echo path no longer exists due to the switching of the line. For this reason, the echo canceller is not always in a state where the impulse response can be estimated correctly.
In order to cope with the above described case, the echo canceller is usually provided with an internal limiting function for constantly carrying out a stable operation by judging the situation. However, the limiting function cannot always cope with all situations, and the estimation of the impulse response may become impossible due to some reason. When the adaptive operation continues for a long time in the state where the impulse response cannot be estimated, the calculation error is gradually accumulated and may eventually reach divergence.
When the above described divergence phenomenon occurs, the echo cancellers 43 and 44 shown in FIG. 2 form a closed loop. As a result, once the filter coefficients diverge, it is extremely difficult to automatically reset the echo canceller to the normal operation.
The divergence phenomenon occurs in the following case. That is, since the echo canceller carries out a complex process in real time, a digital signal processing circuit is usually used for carrying out the complex process. This means that digital signals are processed. On the other hand, analog signals are transferred in the telephone network.
Accordingly, a conversion circuit for converting the analog signal into the digital signal is required at each echo canceller. In most cases, a PCM coder/decoder is used as the conversion circuit as shown in FIG. 3. In FIG. 3, those parts which are the same as those corresponding parts in FIG. 2 are designated by the same reference numerals, and a description thereof will be omitted. In FIG. 3, a PCM coder/decoder 435 is coupled to the echo canceller 43, while a PCM coder/decoder 445 is coupled to the echo canceller 44. The PCM coders/decoders 435 and 445 respectively use as the output thereof a non-linear 8-bit signal such as a code according to the .mu. law and the maximum value is restricted.
At the echo canceller 43, for example, the transmitting side input signal is the output signal of the PCM coder/decoder 435 and the maximum level of the transmitting side input signal will not exceed the maximum output value of the PCM coder/decoder 435. On the other hand, since the echo replica is the estimated echo, the echo replica will not exceed the maximum output value of the PCM coder/decoder 435 during a normal operation.
For example, when the echo canceller 43 carries out the digital signal processing using the non-linear 8-bit output signal of the PCM coder/decoder 435, the non-linear 8-bit output signal is first converted into a linear signal before carrying out the signal processing. When making this conversion, the non-linear 8-bit output signal of the PCM coder/decoder 435 is expanded into a 14-bit signal. Furthermore, when carrying out the echo cancelling process in the echo canceller 43, the calculations are in many cases carried out in 14 bits or more so as to ensure a satisfactory calculation accuracy. In such cases, when the estimation of the echo path is not carried out correctly, the calculated echo replica signal may exceed the maximum output value of the PCM coder/decoder 435.
As a result, the transmitting side output signal corresponding to the residual echo may exceed the maximum output value of the PCM coder/decoder 435. However, even in this case, the other echo canceller 44 is unaffected if the transmitting side output signal is again converted into 8 bits in the PCM coder/decoder 435 before being transmitted to the other echo canceller 44.
But when a high efficiency encoder is used in place of the transmission line, for example, the transmitting side output signal is transmitted to the other echo canceller 44 with the value which exceeds the maximum output value of the PCM coder/decoder 435.
Consequently, the value which exceeds the maximum output value of the PCM coder/decoder 435 is input to the other echo canceller 44 as the receiving side input signal. When this value is output to the echo path, the value is suppressed by the PCM coder/decoder 445. This echo path is formed from the other echo canceller 44 to the transmitting side of the echo canceller 43 via the PCM coder/decoder 445, the hybrid transformer 42 and the PCM coder/decoder 445. The above described suppression at the PCM coder/decoder 445 causes non-linearity between the receiving side input signal and the transmitting side input signal of the other echo canceller 44, and it becomes impossible to carry out a correct echo cancelling operation. Furthermore, the non-linearity may cause the divergence of the other echo canceller 44.
When the above described divergence phenomenon occurs due to some reason, a trouble is introduced thereby in the communication. Conventionally, there is a problem in that the divergence phenomenon can only be eliminated by manually resetting the echo canceller depending on the operator's judgement.